1. Technical Field
The present disclosure relates to a device that encodes a voice signal and an audio signal (hereinafter referred to as a voice signal and the like) and a device that decodes the voice signal and the like.
2. Description of the Related Art
A voice encoding technology that compresses the voice signal and the like at a low bit rate is an important technology that realizes efficient use of radio waves and the like in mobile communication. In addition, expectations for a higher quality telephone voice have been raised in recent years, and a telephone service with enhanced realistic sensation has been desired. In order to realize this, it is sufficient that the voice signal and the like having a wide frequency band is encoded at a high bit rate. However, this approach contradicts efficient use of radio waves or frequency bands.
As a method that encodes a signal having a wide frequency band at high quality at a low bit rate, there is a technique that reduces the overall bit rate by dividing a spectrum of an input signal into two spectra of a low-band part and a high-band part, and by replicating a low-band spectrum and transposing a high-band spectrum with the replicated low-band spectrum, that is, by substituting the low-band spectrum for the high-band spectrum (Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2001-521648). In this technique, encoding is performed by allocating a reduced number of bits by performing the following process as a basic process: encoding a low-band spectrum at high quality by allocating a large number of bits and replicating the encoded low-band spectrum as a high-band spectrum.
If the technique disclosed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2001-521648 is used without any modification, a signal having a strong peak feature seen in the low-band spectrum is replicated as is to the high band. Thus, noise that sounds like a ringing bell is generated, reducing subjective quality. Accordingly, there is a technique that uses a low-band spectrum with an appropriately adjusted dynamic range, as a high-band spectrum (International Publication No. 2005/111568).
In the technique disclosed in International Publication No. 2005/111568, the dynamic range is defined by taking into account all components making up the low-band spectrum. However, the spectrum of a voice signal and the like includes a component having a strong peak feature, i.e., a component having a large amplitude (tonal component), and a component having a weak peak feature, i.e., a component having a small amplitude (non-tonal component). The technique disclosed in International Publication No. 2005/111568 makes evaluation by taking into account all components including both of the above components and therefore does not always produce the best result.